2. BIRTH OF TIDAL DWARF GALAXIES: MODELS, SIMULATIONS

Observations give some clues on the formation mechanism of tidal
dwarfs. In the young TDGs observed so far, the atomic hydrogen makes the
bulk of their mass. Therefore gas should play a key role. On the
theoretical side, several scenarios have been proposed, supported by
various types of numerical simulations:

Local
gravitational instabilities in the stellar component. Simulations of
mergers which only include the stellar component are apparently able to
produce along tidal tails gravitational bound stellar objects, some
reaching the mass of dwarf galaxies
[1].
However it has been claimed that
they might in fact be artifacts of the N-body simulations
[30].

Local
gravitational instabilities in the gaseous component. In simulations
which include the gas component, real massive gas condensations may
locally grow in the tails and form objects similar to TDGs
[30].

Ejection of
Jeans-unstable gas clouds. Due to the increased velocity dispersion
induced by galaxy-galaxy interactions, the Jeans mass of the individual
cloud complexes increases in the outer disks of the parent
galaxies. They are then pulled out by tidal forces, become unstable and
collapse when reaching large galacto-centric distances
[11].

A top-down
kinematical scenario. The global tidal field of galaxies with
extended dark matter halos can efficiently carry away from their disk a
large fraction of the gas, while maintaining its surface density to a
high value
[9].
In fact tidal forces
contribute to stretch the gas only at low galacto-centric distances,
i.e. at the base of the tail. As a result, gas accumulates near the tip
of tidal tail, and then collapses and fragments, through a process
apparently opposite to the bottom-up one favored with the Cold Dark
Matter model for the building up of classical galaxies.

The fully compressive mode of tidal forces. At locations where
tidal forces are compressive rather than destructive, star / cluster
formation may be triggered and/or already formed stellar objects, such
as TDGs, may be protected from disruption
[24].

Merger
between Super-Star-Clusters. SSCs with a range of masses may be
formed in mergers. Some of them might merge to reach the mass of dwarf
galaxies
[12].
The TDGs born that way would then resemble the Ultra Compact Dwarf
Galaxies (UCDs) identified in nearby groups and clusters of galaxies.

The variety of proposed scenarios tells how much having ad-hoc initial
conditions and all necessary ingredients in the simulations is
important: if the dark matter halo is truncated in the numerical
simulations (to lower their computational cost), scenario (4) will not
work; scenarios (2)-(5) require proper treatment of the gaseous
component, including feedback. Scenario (6) needs high resolution, so
as to resolve Super Star Clusters. Fig. 2
presents one of such simulations fulfilling
most of these criteria. The numerical model used a total of 36 million
particles, including 12 million "sticky" particles for the gas
component, and minimal grid cell size of 32 pc
[6].
The production of star clusters with masses down to 105
M was
directly resolved in these simulations. The mass spectrum of objects
produced during the merger seems to be bimodal. Two distinct families
arise: (a) compact SSCs, with masses less than 108
M which seem
pressure supported and may be the progenitors of globular clusters; (b)
extended objects with masses above 108
M which are
usually supported by rotation. The latter have the properties of
observed Tidal Dwarf Galaxies. Thus TDGs are not simply the high mass
end of SSCs, a conclusion that was also reached from the analysis of
HST images
[16].
Furthermore,
analyzing snapshots of the simulation for a period of one Gyr, we found
no evidence that the latter evolve into the former, via merging. The
TDG progenitors are visible soon after the first encounter, in the
outskirts of the colliding galaxies, at a time when the tidal tails
have not yet completely unfolded. They quickly collect all their
building material. After about 100 Myr, their mass is
stabilized. Rotational support appears as well very early on. Only a
few massive objects are formed later on within the tidal tails. Note
however that star-formation and gas feedback are not properly handled
with the sticky particles used in these simulations. Investigations may
now be carried using fully hydrodynamical simulations
[26].

Figure 2. Formation of tidal dwarf galaxies
in high resolution numerical simulation of a major merger
[6].
Two snapshots are shown, resp. after the first encounter and the merger
(Belles et al., in prep).